With the increasing requirement of the power system for efficiency, power density, reliability, installing convenience, the power device develops from the discrete devices to the modular devices. The power modularization becomes an important trend in the power electronics industry. Due to a number of semiconductor chips and some logic, control, detection and protection circuits being integrated therein, the power device can be conveniently applied. The system volume and the development time are decreased, and the reliability of the system is greatly improved.
Due to the harsh environment for product application, the power electronic product may have structure failures after long working hours. Thus more and more attention is paid to improving the reliability of the power electronic product. The optimal design of the structural stress is important for improving the reliability of the power electronic product.
In the related art, the package terminal of the package module of the power device is shown in FIGS. 1 and 2A-2C. The package terminal includes an end portion 91 and a base 93. The base 93 of the package terminal is connected through the solder 94 to the substrate 95, such as a Direct Bonded Copper (DBC) substrate, a Direct Bonded Aluminum (DBA) substrate, a metallized ceramic substrate or a Low Temperature Co-fired Ceramic (LTCC) substrate. The end portion 91 is bonded to the PCB 96. Thereby the signal transmission between the power device and the PCB 96 is achieved. By the impact of the structure and the application of the package terminal, the failure risks of the package terminal in the related art are shown as follow.
Firstly, due to the different coefficient of thermal expansion of various materials in the application, the thermal stress is generated in the package module. Because strength of the material of soldering interface is lower, the fracture of the soldering layer between the package terminal and the substrate becomes one of the major failure forms.
Secondly, due to the manufacture tolerance and the location deviation of the terminal, the fracture of the soldering layers can also be generated from the continuous mechanical interference force after assembly. In addition, in the storage and service process, the damage at the soldering layers can also be generated by various external mechanical vibrations.
In order to overcome the above drawbacks, many technical solutions are proposed. The Z-shape terminal is shown in FIGS. 2A-2C. The Z-shape bend is applied to the root (i.e., the bent portion 92) of the package terminal to release the stress. The stress is significantly reduced after the design of FIGS. 2A-2C is applied.
The experimental value of the mechanical property of the Z-shape terminal is shown in FIG. 3, in which the horizontal axis shows the deflection (mm), and the vertical axis shows the normal force (kgf). In the package terminal (e.g. Z-shape terminal) in the related art shown in FIG. 3, the elastic property in the two directions perpendicular to each other is very different (Dir.1 indicates the bending direction; Dir.2 indicates the direction perpendicular to the bending direction). As shown in FIG. 3, the solid line presents the mechanical property in Dir.2 direction, the dashed line presents the mechanical property in Dir.1 direction, the elasticity value of K (the ratio value of the vertical axis value and the horizontal axis value) is in a range of 0.080 to 0.394 kgf/mm, which means the stress-releasing effect in Dir.1 direction is better than that in Dir.2 direction.
Based on the above experimental value, as the direction of the external force is highly uncertain during the actual application, the installing direction of the package terminal is thus randomly allocated in general. The obvious drawback is that when the external force direction is concordant with the direction perpendicular to the bending direction, the soldering point generally withstands a relatively large stress, which may cause the risk of failure. Thus, it is necessary to further improve the reliability of the package terminal in the related art.